1. Field of the Invention
The present invention relates to a process for the granulation of sodium metasilicate, and, more especially, to the preparation of sodium metasilicate having a narrow range of granulometric size distribution and good resistance to attrition or "dusting". The invention also relates to a storage-stable sodium metasilicate which is unreactive with both perfumes and non-ionic compounds, e.g., surfactants. In particular, the invention relates to anhydrous sodium metasilicate comprising 2 to 6% water in the final product, principally in the form of the sodium metasilicate pentahydrate (55 to 80% of the 2 to 6% of this water).
2. Description of the Prior Art
The problem with which this art has been faced as regards effective granulation with concomitant resistance to attrition is made difficult by two requirements, one of which demanding good abrasion resistance and the other consisting of granulating an anhydrous metasilicate powder while employing but small amounts of binders. Known responses to this dual requirement have been contradictory. In actual fact, if good resistance to attrition be desired, large amounts of binder must be used, which is difficult to reconcile with the use of large quantities of metasilicate.
Processes to prepare granules of a variety of substances which are solid at ordinary temperatures and soluble in water have long been proposed to the art. One of such processes consists of spraying droplets of a heat liquefied solution by means of suitable atomizing device, through a tower containing a drying or cooling gas. This process has been deemed pelletizing or "prilling". However, this process displays the disadvantage of difficult operation because of many technical problems; mass production requires long retention times and, thus, large scale equipment.
In French Pat. No. 1,100,817, it has been proposed to atomize a paste under pressure in a rotating furnace, in parallel currents heated by flow of hot gas, and equipped with means to advance the product, such as small blades.
And in French Pat. No. 1,351,668 [U.S. Pat. No. 3,227,789], a process for coating in the form of onionskin-like layers is described; the operations of granulation and drying taking place virtually at the same time. According to this process, the metasilicate particles are introduced into a coating zone at a temperature in excess of 130.degree. C. in order to establish a moving bed of crystallized particles, then a solution of anhydrous sodium metasilicate is atomized so as to effect formation of a metasilicate coating layer, which is then heated to eliminate the water. It suffices to subsequently repeat this operation as many times as are considered necessary to obtain the desired product. In these cases, a rotating furnace equipped with blades is utilized to raise the material and form a moving bed thereof, thus contributing to the operation of the process.
Cf. U.S. Pat. Nos. 3,208,822 and 3,868,227; Canadian Pat. No. 592,240; and Chemical Engineering Process, 69, No. 2, pp. 62-66 (1973).